The proposed research seeks to understand the ways in which antigen molecules can either stimulate or inactivate lymphocytes, producing a state either of immunity or immunologic tolerance. The working hypothesis is that the degree of maturity in the differentiation pathway is a key variable in determining the outcome of an encounter between antigen and lymphocyte. Various monoclonal antibodies directed against differentiation antigens on the lymphocyte surface will be used in combination with the fluorescence-activated cell sorter to identify cells of the B lymphocyte lineage which precede the functional B cell. Specialized tissue culture techniques will be developed to allow the clonal growth of single B cells. The effects of monovalent and multivalent tolerogens on cellular subsets will be studied both in vitro and in vivo, with emphasis on the subsequent enumeration, by limiting dilution studies, of the effects of such encounters on the capacity of cells to be activated by antigen, antigen plus T lymphocyte help, or various mitogens. The approach will be extended to study various T lymphocyte subsets including "helper" T cells, "suppressor" T cells and precursors of cytotoxic T cells.